EFFECT OF PH SOLUTION ON THE WATER ABSORBENCY OF

SUPERABSORBENT POLYMER COMPOSITE

AYUNI ATHIRAH BINTI AZMAN

Thesis submitted in fulfilment of the requirements

for the award of the degree of

Bachelor of Chemical Engineering

Faculty of Chemical & Natural Resources Engineering

UNIVERSITI MALAYSIA PAHANG

JANUARY 2013

vi

ABSTRACT

Nowadays, superabsorbent polymer composite (SAPC) has been world widely known

because of its capacity in contributing beneficial applications in daily life. In this study,

Poly Oil Palm Empty Fruit Bunch-co-Acrylamide superabsorbent polymer composite

(OPEFB-AM-SAPC) was synthesized by solution polymerization of the Acrylamide

(AM) monomer onto OPEFB fibre using Ammonium Persulphate (APS) and N, N-

methylene bisacrylamide (MBA) which act as an initiator and crosslinker, respectively.

The effects of different pH solution and filler amount towards water absorbency have

been identified by studying the optimum condition of each parameter towards water

absorbency capacity of polymer. For a parameter of pH solution, the maximum water

absorbency was observed at pH 4 for fixed filler amounts. Meanwhile, for the effect of

different filler loading, the optimum water absorbency of OPEFB-AM-SAPC was

achieved at 2.5 wt% of filler loadings which reveals the well-organized loosely

polymeric structure with multiple porous structures that suitable for penetration of water

into the polymeric network. These multiple porous structures lead for high water uptake

within the network. On the other hand, the characterizations of OPEFB-SAPCs have

been carried out by using Fourier Transform Infrared Spectroscopy (FTIR),

Thermogravimetric Analysis (TGA) and Field Emission Scanning Electron Microscopy

(FESEM). The thermogravimetry analysis result of OPEFB-SAPC at 2.5% filler loading

indicate that the SAPC shows a three stage degradation, which unlike the dense

unorganized rigid structure been exhibited by the 12.5 wt% filler loading. Meanwhile,

FTIR analysis shows OPEFB-SAPC (2.5 wt %) has sharp peak of bonding curves

compared to OPEFB-SAPC (12.5 wt%).

vii

ABSTRAK

Pada masa sekarang, penyerap polimer gel (SAPC) telah diketahui secara umum dan

meluas kerana kebolehannya yang menyumbangkan banyak kelebihan dalam kehidupan

seharian. Di dalam penyelidikan ini, penyerap polimer gel (SAPC) bedasarkan tandan

kosong buah kelapa sawit (EFB) disentesiskan menggunakan kaedah pempolimeran

cantuman akrilamida (AM) monomer ke atas tulang belakang OPEFB dengan

ammonium persulfat (APS) sebagai pemangkin dan N'N'-metilenabisakrilamida (MBA)

sebagai pemautsilang dalam membantu proses. Kesan kuantiti pengisi (filler) dan kesan

larutan pH yang berbeza terhadap kebolehan daya serap air dipelajari untuk menentukan

keadaan kuantiti maksimum kebolehan daya serap air OPEFB-SAPC. Kebolehan daya

serap air paling tinggi untuk larutan pH adalah 4 untuk kuantiti pengisi (filler) yang

tetap. Selepas itu, daya serap air OPEFB-SAPC paling maksimum adalah 2.5 wt%.

Manakala, analisa struktur kimia OPEFB-SAPC dianalisis menggunakan spektroskopi

FTIR, TGA dan FESEM.

viii

TABLE OF CONTENTS

Page

SUPERVISOR’S DECLARATION

STUDENT’S DECLARATION

ACKNOWLEDGEMENTS

ABSTRACT

ABSTRAK

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF SYMBOLS

LIST OF ABBREVIATIONS

ii

iii

v

vi

vii

viii-ix

x

xi

xii

xiii

CHAPTER 1 INTRODUCTION

1.1 Background of Study

1.2 Problem Statement

1.3 Objectives

1.4 Scope of Study

1.5 Significance of Study

1- 4

4

4-5

5

5

CHAPTER 2 LITERATURE REVIEW

2.1 Superabsorbent Polymer Composite (SAP)

2.2 Natural based SAP

2.3 Oil Palm Empty Fruit Bunch

2.4 Technique of Polymerization

2.5 General Reaction And Mechanism of SAPC

2.6 Effect of Filler (OPEFB) Amount

2.7 Effect of pH Solution

6-7

8-9

9-11

11-12

12-15

15

16

ix

CHAPTER 3 METHODOLOGY

3.1 Materials and Solvents

3.2 Apparatus and Equipment

3.3 Research Design

3.4 Sample Preparation

3.4.1 Pre-Treatment of Oil Palm Empty Fruit Bunch (OPEFB)

3.4.2 Preparation of Oil Palm Empty Fruit Bunch Based

Superabsorbent Polymer Composites (OPEFB-SAPC)

3.4.3 Preparation of Pure Superabsorbent Polymer Composites

3.4.4 Preparation of Buffer Solution

3.5 Water Absorbency Measurement

3.6 Characterization

3.6.1 Fourier Transform Infrared Spectrometer (FTIR)

3.6.2 Thermal Gravimetric Analysis (TGA)

3.6.3 Field Emission Scanning Electron Microscopy (FESEM)

17

17

18

19

19

20

21

21

22

23

23

23

23

CHAPTER 4 RESULTS

4.1 Water Absorbency testing in pH Solution

4.2 Effect of Filler on Water Absorbency

4.3 Fourier Transform Infrared Spectroscopy (FTIR)

4.4 Thermogravimetric Analysis (TGA)

24-26

27-29

30-32

33-35

CHAPTER 5 CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion

5.2 Recommendations

REFERENCES

APPENDICES

36

37

38-41

42

x

A1 Summary data of water absorbency testing

A2 Statistical Analysis

B1 Fourier Transform Infrared Spectroscopy (FTIR) for OPEFB-

SAPC (2.5 wt %)

B2 Fourier Transform Infrared Spectroscopy (FTIR) for OPEFB-

SAPC (12.5 wt %)

B3 Thermal Gravimetric Analysis for OPEFB-SAPC (2.5 wt %)

B4 Thermal Gravimetric Analysis for OPEFB-SAPC (12.5 wt %)

C1 Chemicals and OPEFB filler

C2 Polymerization process apparatus

C3 Sieve shaker

C4 pH meter

C5 Analytical balance

D1 Fresh OPEFB-SAPC

E1 Tea-bag Method

F1 FTIR

F2 TGA

F3 FESEM

42

43-44

44

46

47

48

49

49

50

50

51

52

53

54

54

55

xi

LIST OF TABLES

Table No. Page

2.1 Water absorbency of absorbent materials 7

4.1 Intensity value at same wavenumber 33

xii

LIST OF FIGURES

Figure No. Page

2.1 Comparison of dry SAP with swollen SAP and schematic

of the SAP swelling

7

2.2 Oil palm empty fruit bunch fibrous 10

2.3 Water absorption (%) of oil palm OPEFB / Jute reinforced

hybrid composites

10

2.4 Synthesis of OPEFB-g-PAAm SAP 11

2.5

2.6

2.7

2.8

The mechanism in preparation of SAP

Structures of some of the cross-linking agents

Preparation of poly(acrylamide/maleic acid) hydrogel

(PAM), and Poly (acrylamide/maleic acid)-sepiolite

composite hydrogel (PAMS). (A Acrylamide, M maleic

acid, NNMBA N,N0- methylenebisacryl amide, S

sepiolite.

The effect of filler amount towards water absorbency

11

13

14

15

3.1 Research Design 18

4.1 Graph of water absorbency versus pH solution 24

4.2 Graph of water absorbency versus filler loading 27

4.3 2.5 wt% of filler loadings at x2000µm magnification 29

4.4 2.5 wt% of filler loadings at x1000µm magnification 29

4.5

4.6

4.7

2.5 wt% of filler loadings at x500 µm magnification

FTIR spectra of (a) pure SAPC, (b) OPEFB-SAPC (2.5

wt%) and (c) OPEFB-SAPC (12.5 wt%)

TGA curves of pure SAPC, OPEFB-SAPC (2.5 wt%) and

OPEFB-SAPC (12.5 wt%) of filler loadings

29

30

33

xiii

LIST OF ABBREVIATIONS

AM Acrylamide

APS Ammonium Persulphate

FESEM Field Emission Scanning Electron Microscopy

FTIR Fourier Transform Infrared Spectroscopy

HCl Hydrochloric Acid

MBA N,N‟-methylenebisacrylamide

NaOH Sodium Hydroxide

N2 Nitrogen

OPEFB Oil Palm Empty Fruit Bunch

SAP Superabsorbent Polymer

SAPC Superabsorbent Polymer Composite

SPAN Starch-graft-polyacrylonitrile

TGA Thermogravimetric Analysis

1

CHAPTER 1

INTRODUCTION

1.1 BACKGROUND OF STUDY

Superabsorbent are three-dimensional a cross-linked network of hydrophilic

polymers that can absorb large quantities of water, saline or physiological solutions

while the absorbed solutions are not removable even under pressure (Hossein et al.,

2011). Based on study by Gadallah et al., (2012), to function as an absorbent for

aqueous fluids, a polymer must have certain properties which is must be hydrophilic

and the polymer must swell in aqueous fluids but must not dissolve. According to

Zohuriaan and Kabiri, (2008), the synthesis of the first water-absorbent polymer goes

back to 1938 when acrylic acid (AA) and divinylbenzene were thermally polymerized

in an aqueous medium. In the late 1950s, the first generation of hydrogels was

appeared. These hydrogels were mainly based on hydroxyalkyl methacrylate and

related monomers with swelling capacity up to 40-50%. They were used in developing

contact lenses which have made a revolution in ophthalmology. The first commercial

SAP was produced through alkaline hydrolysis of starch-graft-polyacrylonitrile

(SPAN). The hydrolyzed product (HSPAN) was developed in the 1970s at the Northern

Regional Research Laboratory of the US Department of Agriculture. Expenses and

inherent structural disadvantage (lack of sufficient gel strength) of this product are

taken as the major factors of its early market defeat. Commercial production of SAP

began in Japan in 1978 for use in feminine napkins.

Because of the superior properties of SAPs, they have found extensive

applications such as disposable diapers, feminine napkins, drug delivery systems, and

soil for agriculture and horticulture. For the majority of applications, the

2

superabsorbent polymers have to possess high absorption capacity and elevated

swelling rate and show a strong swollen gel. Hydrogels with high mechanical strength

are required in some applications such as artificial cartilage, controlled drug delivery,

hygiene and agricultural uses (Hossein et al., 2011). Recently, based on research from

Xie and Wang (2009), the usage of superabsorbent as water managing materials for the

renewal of arid and desert environment has attracted great attention as they can reduce

water consumption for irrigation, improve fertilizer retention in soil, lower the

mortality rate of plants, and increase plant growth rate.

Furthermore, SAPs are used also as scaffolds in tissue engineering where they

may have human cells in order to repair tissue. Superabsorbent polymers have the

ability to sense environmental changes, like changes of pH and temperature.

Hydrophilic networks that are responsive to some molecules, such as glucose or

antigens can be used as biosensors as well as in drug systems, disposable sanitary

products (for example, diapers, incontinence articles, feminine hygiene products,

airlaids and absorbent dressings), and in controlled release drugs. Superabsorbent

polymers were also employed in various applications, such as household articles,

sealing materials, humectants for agricultural products for soil conditioning, oil-

drilling, anti-condensation coatings, water-storing materials in agriculture, absorbent

paper products, bandages and surgical pads, pet litter, wound dressings, and as

chemical absorbents. Furthermore, they are used in food packaging applications (Jaber,

2012).

In general, there are two types of SAP that available in the market which are

synthetic (petrochemical-based) and natural. The graft copolymerization of vinyl

monomers on polysaccharides is the example of the natural based SAP where usually

been prepared through addition of some synthetic parts onto the natural substrates. The

greatest volume of SAP comprises full synthetic or of petrochemical origin which

produced from the acrylic monomers, frequently used are acrylic acid (AA) and acrylic

amide (AM) (Zohuriaan-Mehr and Kabiri, 2008). This superabsorbent polymer can be

prepared by various techniques such as bulk polymerization, suspension-inverse

suspension polymerization and polymerization by irradiation. However, the frequently

common method used for SAP preparation is solution polymerization technique which

3

is a free-radical initiated polymerization of acrylic acid (AA) and its salts, acrylic amide

(AM) with a cross-linker. Before or after the polymerization step, the carboxylic acid

groups of the product are partially neutralized. There are few types of initiation often

carried out by reaction of a reducing agent with an oxidizing agent (redox system) or

chemically with free-radical azo or peroxide thermal dissociative species or.

Additionally, radiation is sometimes used for initiating the polymerization. The

solution polymerization of AA and AM with a water-soluble cross-linker, e.g., MBA in

an aqueous solution is a straight forward process. The reactants are dissolved in water

at desired concentrations, mostly about 10-70% and a fast exothermic reaction yields a

gel-like elastic product. Then, the product is dried and sieved in order to obtain the

required particle size (Zohuriaan-Mehr and Kabiri, 2008). Based on study by

Kiatkamjornwong (2007), the major advantage of solution polymerization is the

presence of solvent serving as a heat sink. A great variety of hydrogels has been

synthesized where the SAP can be made pH-sensitive or temperature-sensitive by using

this method as well.

Currently, material‟s biodegradability has been widely focused on due to the

renewed attention towards environmental protection issues. Approximately, 90% of

superabsorbent materials are used in disposable articles which most of them are

synthetic polymers that are poor in degradability. Poor degradability will eventually

leads to the environmental problem. However, according to previous work (Zhang et

al., 2007); the degree of degradability of this superabsorbent polymer could be

improved by incorporation of biodegradable and renewable natural sources such as

starch, cellulose, and chitosan. It was believed that incorporation of biodegradable

element is a convenient way to improve biodegradability of corresponding

superabsorbent materials. Natural- based SAP polymers have attracted much attention

in medical and pharmaceutical fields because of their non-toxicity, biocompatibility

and biodegradability (Sadeghi, 2012). Moreover, the introduction of low cost inorganic

fillers such as natural filler into a polymer matrix could increase their strength and

stiffness properties as well as reduced the production cost (Hossein et al., 2011).

Therefore, this study has been carried out by utilization of natural filler in order

to improve the absorbency capacity and their strength. For examples, in Malaysia,

4

agricultural waste materials such as oil palm wastes, paddy straw and rice husk are

increasing each year leading to disposal problem and need to manage in a proper way.

The conventional method of burning OPEFB for disposal purpose often creates

environmental problems in that it generates severe air pollution. Thus, economic

utilization of OPEFB in turning its abundant supply from oil palm industry by-products

into value-added products will be beneficial. Therefore, grafting of vinyl monomer

such as AA or AM onto OPEFB backbone may be used to modify and improve various

properties in the original vinyl polymer such as elasticity, absorbency, ion exchange

capabilities, thermal resistance and hydrophilicity. The synthesized SAPC has benefited

the system by enhancing the swelling ability while reducing the production cost, more

environmental friendly and accelerate the generation of new materials for special

applications (Hashim and Jamaludin, 2011).

1.2 PROBLEM STATEMENT

Nowadays, development of SAP has been improved from time to time. SAPC

made from synthetic polymers possess good characteristics but it is not environmental

friendly since it contains toxicity and non-degradability. SAP based on acrylic acid and

acrylamide are poor in degradability in application of agriculture and horticulture. As

an alternative way, OPEFB used as the filler in SAPC and lower the cost production.

Additionally, SAPC that will be produced is biogradable and easy to dispose so it does

not pollute the surrounding environment. This SAPC is also able to absorb water higher

than synthetic SAPC with proved from recently research that had been going through.

Thus, OPEFB based on SAPC may become a new invention to be used in widely

agriculture, sanitary goods as well as in horticulture field.

1.3 OBJECTIVE

The main objectives of this research is to study the optimum conditions of oil

palm empty fruit bunch (OPEFB) based on the superabsorbent polymer composite by

determining:

a) Effect on different of pH solutions towards water absorbency.

5

b) Effect on amount of filler towards water absorbency.

1.4 SCOPE OF STUDY

The effects of filler amount and effect of different pH solution towards water

absorbency have been studied to determine the optimum condition for water

absorbency capacity of OPEFB-SAPC. A few parameters required to be controlled in

this research which is by fixing pH solutions at pH 2 up to pH 10 while varying amount

of filler at range of 5 wt% to 12.5 wt%. In this research, SAPC were synthesized by

using solution polymerization with acrylamide (AM) was used as the monomer,

ammonium persulphate (APS) was used as the initiator as well as N‟N‟-

methylenebisacrylamide (MBA) as a crooslinker. In sample preparation, three flasks

equipped with a stirrer, condenser, thermometer, and nitrogen line were used. The

samples were characterized by using FTIR (Fourier Transform Infrared) spectroscopy

to indicate functional groups, TGA (Thermal Gravimetric Analysis) to indicate thermal

stability of samples and FESEM (Field Emission Scanning Electron Microscope) to

examine morphology of superabsorbent polymer composite (SAPC). Finally, the tea-

bag method was used to measure the amount of water absorbency.

1.5 SIGNIFICANCE OF STUDY

Superabsorbent polymer composite (SAPC) from oil palm empty fruit bunch

(OPEFB) become new materials to be used in the application of agriculture, sanitary

goods and horticultural. The significant of this research can reduce overall cost to

produce SAPC with the same quality as superabsorbent polymer synthetic since

OPEFB is residue where it can be found easily at the palm oil mill around Malaysia.

The addition of this research is the SAPC produced has biodegradable element and

reducing the environment problems and protect the earth. The swelling ability of this

SAP also increases compare to the synthetic SAP which has been proved by the

recently research that had been done.

6

CHAPTER 2

LITERATURE REVIEW

2.1 SUPERABSORBENT POLYMER COMPOSITE (SAP)

According to Zohuriaan-Mehr and Kabiri (2008), superabsorbent polymers are

slightly cross-linked hydrophilic polymers with a three-dimensional network structure

which are capable of absorbing and retaining large amounts of aqueous fluids even

under some pressure. Desired features of superabsorbent polymer (SAP) are high

swelling capacity, high swelling rate, and good strength of the swelling gel. SAP

hydrogels also known as polymeric materials which exhibit the ability of swelling in

water and retaining a significant fraction of water within their structure without

dissolving in water or aqueous solution (Brannon-Peppas and Harland, 1990; Buchholz

and Graham, 1998). There are two types of SAP which are synthetic (petrochemical-

based) and natural. The graft copolymerization of vinyl monomers on polysaccharides

are the example of the natural based SAP where usually been prepared through addition

of some synthetic parts onto the natural substrate. Absorption capacity of common

hydrogels usually not more than 100% (1g/g) but superabsorbent hydrogels can absorb

deionized water as high as 1000-100000% (10-1000g/g) which can be seen on Figure

2.1. (Omidian et al., 2004).

7

Figure 2.1: Comparison of dry SAP with swollen SAP and schematic of the SAP

swelling

Moreover, after water absorption and swelling, SAP particle shape (granule, fibre, film,

etc) has to be basically preserved, which the swollen gel strength should be high

enough to prevent a loosening, mushy, or slimy state. Traditional absorbent materials

such as tissue, papers and polyurethane forms unlike SAP, will lost most of their

absorbed water when they are squeezed. Comparisons of water absorptiveness of some

common absorbent materials with a typical sample of a commercially available SAP

nowadays are shown in the Table 2.1.

Table 2.1: Water absorbency of absorbent materials

Absorbent Material Water Absorbency (wt %)

Whatman No. 3 filter paper 180

Facial tissue paper 400

Soft polyurethane sponge 1050

Wood pulp fluff 1200

Cotton ball 1890

Superab A-200a 20200

8

2.2 NATURAL BASED SAP

Kiatkamjornwong et al. (2010), used cassava starch for polymer substrate,

acrylamide, AM as a grafting monomer, potassium persulfate, KPS as initiator and

N,N‟-Methylenebisacrylamide (MBA) as crosslinker. The water absorbency of cassava

starch-g-polyacrylamide which has been saponified in this experiment was 605 g/g.

However, when the testing for the comparison of inorganic filler, the bentonite clay

SAP showed the highest water absorption of 730 g/g among the China clay, 650 g/g

and silica, 310 g/g. From the study, it shows that the pure SAP without inorganic filler

still can produce high water absorbency of 605 g/g but when filler was added it helps in

improving the capacity of the water absorbency.

Soy and fish proteins are converted to SAP through modification by

ethylenediamine tetraaceticdianhydride (EDTAD). The amino groups of the protein

was crosslinked by glutaraldehyde to produce SAP. The dry gel of SAP was capable to

absorb 80-300 g of deionized water/g after centrifugating at 214 g. The water

absorbency capabality of SAP was depending on the extent modification, protein

structure, cross link density, protein concentration and environmental conditions like

pH, ionic strength and temperature (Hwang and Damodran, 1996). This research show

that the protein after modification could be used as polymer substrate and produce SAP

with high water absorption.

Starch phosphate-graft-acrylamide or attapulgite superabsorbent composite was

prepared by graft-copolymerization among starch phosphate, acrylamide, and

attapulgite in aqueous solution (Raju et al., 2005). The factors influencing water

absorbency of the superabsorbent composite such as the molar ratio of NaOH to AM

and the amount of starch phosphate and attapulgite were studied. Hence, the

superabsorbent composite achieved the highest equilibrium water absorbency of

1268 g/g when the molar ratio of COO−, COOH, and CONH2 is 10:3:11, the weight

ratio of AM to starch phosphate is 5:1, and 10 wt% attapulgite was incorporated. In this

research, the results show that the phosphorylation of starch and the introduction of

attapulgite could greatly improve equilibrium water absorbency superabsorbent

composite.

9

The effects of vermiculite content on water absorbency were studied by Zheng

et al. (2007), in a series of superabsorbent composites that were synthesized by

copolymerization reaction. This copolymerization reaction was occurred between a

partially neutralized acrylic acid on unexpanded vermiculite (UVMT) micropowder

using N,N′-methylenebisacrylamide (MBA) as a crosslinker and ammonium persulfate

(APS) as an initiator in aqueous solution. They found that the equilibrium water

absorbency increased with increasing UVMT content and the concentration of 20 wt %

clay gave the best absorption of 1232 g/g in distilled water and 89 g/g in 0.9 wt %

NaCl. From the result obtained in this research, it is found that the UVMT helps in

improving the absorbency of water and also saline solution.

2.3 OIL PALM EMPTY FRUIT BUNCH (OPEFB)

According to Shinoj et al. (2011), the lignocellulosic materials are from the

excess of oil palm tree which can be extracted from oil palm fronds, trunks and also

empty fruit bunch. OPEFB is the fibrous mass left after separating the fruits from fresh

fruit bunches where it has 73% fibers among the various source in oil palm tree.

However, these waste materials will cause tremendous environmental problems when

left in field. Furthermore, the additional advantage of natural fiber than glass fiber is

that it can be composted at the end of their life cycle.

Figure 2.2: Oil Palm Empty Fruit Bunch Fibrous

From the research of Jawaid et al. (2010) about the hybrid composites made

from OPEFB/jute fibres, they found that the hydrophilic properties of lignocellulosic

materials and capillary action will cause the intake of water when the samples were

10

soaked into water. It is also observed that the thickness swelling for the pure OPEFB

composite with the value of 9.12 % was the highest water absorption among different

types of composite. The next highest water absorption among the different type

composite is pure OPEFB with the value of 21.39 % which resulted from the high

porosity on the surface of pure OPEFB composite.

Figure 2.3: Water absorption (%) of OPEFB reinforced hybrid composites

Moreover, according to Jawaid et al. (2010), the water absorption behaviour of

the polymer composite depends on the ability of the fibre to absorb water due to the

presence of hydroxyl groups. From their study, it shows that the pure OPEFB has

higher potential than pure jute mate and hybrid composite (OPEFB/jute mate) in water

absorption, which by this reason strengthens the usage of OPEFB as filler in this

research.

2.4 TECHNIQUE OF POLYMERIZATIO N

The polymerization techniques often used in preparing superabsorbent polymer

(SAP) either by solution or suspension polymerization. Each of the techniques has its

own advantages and disadvantage depends on the product been produced. The

mechanism in preparation of SAP was shown in Figure 2.5.

11

Figure 2.5: The mechanism in preparation of SAP

According to Zohurian-Mehr and Kourosh (2008), the solution technique

frequently used for SAP preparation is a free radical initiated polymerization of acrylic

acid (AA) and its salts, acrylic amide (AM) with a cross-linker. Before or after the

polymerization step, the carboxylic acid groups of the product are partially neutralized.

There are few types of initiation often carried out by reaction of reducing agent with an

oxidizing agent (redox system), or chemically with free radical azo or peroxide thermal

dissociation species. The process of AA and AM with a water soluble cross-linker, e.g.,

N‟N‟-methylenebisacrylamide (MBA) in an aqueous is a straight forward process. The

reactants at desired concentration about 10-70 % are dissolved in water and a fast

exothermic reaction yields a gel-like elastic product. Then, the product is dried and

sieved to obtain the required size particles. Based on study by Kiatkamjornwong

(2007), stated that the major advantage of the solution polymerization is the presence of

solvent serving as a heat sink. A great variety of hydrogels has been synthesized where

the SAP can be made pH-sensitive or temperature-sensitive by using solution

polymerization method.

From the research of Zohurian-Mehr and Kourosh (2008), the suspension

polymerization is also referred as inverse suspension because the process is water-in-oil

(W/O) has been chosen. The monomers and initiator are dispersed in the hydrocarbon

phase as a homogenous mixture. Each particle contains all the reactive species when

the initiator dissolves in the aqueous phase and behaves like an isolated micro-batch

12

polymerization reactor. According to Kiatkamjornwong (2007), the mixture is

thermodically unstable and being stabilized by addition of stabilizer. Besides, the SAP

with high swelling ability and fast absorption kinetics is the production of inverse

suspension where it is a highly flexible and versatile technique. The products from the

continuous organic phase are easily removed by filtration or centrifugation.

Furthermore, it is an advantageous method because the products obtained as powder or

microspheres (beads) and grinding is not required.

However, the solution method may often preferred by manufacturers for a

general production of SAP with acceptable swelling properties, the less expensive and

faster techniques rather than suspension techniques.

2.5 GENERAL REACTION AND MECHANISM OF SAPC

The superabsorbent composite, was prepared by graft copolymerization of

acrylic acid onto carrageenan in the presence of a crosslinking crosslinking agent and

powdery kaolin. Ammonium persulfate was used as an initiator. The persulfate is

decomposed under heating and produced sulfate radicals that abstract hydrogen from

one of the functional groups in side chains of carrageenan backbones. So, this

persulfate-saccharide redox system results in active centres capable to radically initiate

polymerization of acrylic acid led to a graft copolymer. Since a crosslinking agent, e.g.

MBA, is presented in the system, the copolymer comprises a crosslinked structure

(Sadeghi et al., 2012).

In addition, other cross-linking agent were also used including 1, 4-butanediol

diglycidyl ether (1, 4-BDGE), and ethylene glycol diacrylate (EGDA). Ethylene glycol

diacrylate was chosen because it is a well-known cross-linking agent that is reported in

the literature as a cross-linking agent for superabsorbent polymers. 1, 4-Butanediol

diglycidyl ether was used for the first time as cross-linking agent for superabsorbent

polymer.

13

Figure 2.6: Structures of some of the cross-linking agents

There are three principal bonding types that are used to bind the polymer chains

together: covalent, ionic, and hydrogen bonds. Two basic methods are used to introduce

covalent crosslinks. First, covalent crosslinks are formed when the major monomers

(e.g., acrylic acid) is copolymerized with a di-,tri-, or tetra – vinyl monomer for

instance N,N-methylenebis(acrylamide), 1,1,1-trimethylolpropanetriacrylate, or as well

as tetraallyloxyethane, in a free radical initiated addition polymerization.

Covalent cross-links are also introduced by reacting the polymer chains with a

di- or tri - functional reagents that reacts with the carboxylic acid groups by means of a

condensation or addition reaction. Second, ionic cross-links are formed by reacting a

polyvalent ion of opposite charge with the charged polymer chains. The crosslink forms

as a result of charge association of the unlike charges. Because the bond is formed by

ion association (charge neutralization) the chemical structure of the cross-linker is less

important in determining the placement of the cross-links compared with covalent

cross-links. If ionic components are present in the liquid to be absorbed, ion exchange

may occur with the ionic cross-links, which may alter the nature of the crosslinks and

the behaviour of the polymer in ways that may be unforeseen. Also because the

interionic reaction is very fast. The incorporation of the crosslink and the resulting

structure of the crosslinked polymer can be difficult to control.

14

The third type of crosslink is the physical crosslink, which is usually formed by

means of hydrogen bonding of segments of one chain with the segments of another

chain is shown (Jaber, 2012).

Figure 2.7: Preparation of poly(acrylamide/maleic acid) hydrogel (PAM), and Poly

(acrylamide/maleic acid)-sepiolite composite hydrogel (PAMS). (A Acrylamide, M

maleic acid, NNMBA N,N0- methylenebisacryl amide, S sepiolite)

Source: (Oztop et al., 2009)

Poly(acrylamide/maleic acid)–sepiolite composite hydrogels were prepared by

free radical crosslinking and copolymerization of acrylamide, sepiolite and maleic acid

with a small amount crosslinker (NNMBA) in aqueous solution. APS and TEMED

were used as the initiator and the accelerator, respectively. At polymerization, the

possible step is a reaction amongst AAm and anionic comonomer, M and crosslinker

molecules by the process of the unpaired electron transfer to the monomeric units, so

that they in turn become reactive. Another monomer or comonomers can be attached

and activated in the same way resulting in a three dimensional network. Sepiolite

molecules can be incorporated into chains simultaneously (Oztop et al., 2009).

15

2.6 EFFECT OF FILLER (OPEFB) AMOUNT

The influences of oil palm empty fruit bunch towards water absorbency give a

strong effect in synthesizing the superabsorbent polymer composites. According to

Shafinaz and Shahrir (2011), small amount of filler (5 wt% of OPEFB) does not

provide enough crosslinking point within the SAPC polymeric network space, thus

decreased the water absorption capacity. However, the increasing of OPEFB filler

contents (10 wt% of OPEFB) enhance the ability of water absorbency due to the OH

molecules on the OPEFB backbone could react with AAm monomer, which benefit the

system by forming a network structure.

Moreover, as further increase in OPEFB amount from 10 wt% to 15 wt%

reduce the ability of water absorbency due to the decreasing in elasticity of SAPC. This

may be attributed to the fact that additional OPEFB fibre in the SAPC system results in

the generation of more crosslink points in the polymeric network. This is because it

contains a lot of hydroxyl groups to form superfluous network point, hence increases

the network density of the composite which it leads to a more difficult permeation of

water into the SAPC system.

Amount of OPEFB (wt %)

Figure 2.8: The effect of filler amount towards water absorbency